medals and awards in SH3 are based on your total performance to date, not performance of the last mission.
I cant recall if it is based on total renown or total tonnage, but either way, if you were just a hair under the required amount to win the knights cross, and on this mission you only manage to patrol the assigned area and sink only a fishing boat... you will still get the medal based on passing the milestone of XXXX amount of renown (or tonnage which ever the case may be)
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